An ideal voltage controlled oscillator (VCO) generates a single-tone output signal at a frequency determined as a linear function of an input control voltage. For such an ideal VCO, the spectrum of the output signal assumes the shape of an impulse. In practice, however, the output signal generated by a VCO includes random fluctuations referred to as phase noise. Phase noise is seen in the spectrum of the output signal as “skirting” around the impulse. In communication systems that use a VCO output signal to up-convert or down-convert a signal, this phase noise can corrupt the resulting frequency-translated signal.
For example, in a received signal, a desired channel centered at a frequency ω0 can be spaced very close to a strong, undesired channel centered at a frequency ω0-Δf. To down-convert the desired channel to baseband, the VCO can be tuned to provide an output signal with a frequency equal to the center frequency ω0 of the desired channel, and the two signals can be mixed. In the ideal case, the VCO output signal consists of a single tone, with no phase noise, at a frequency ω0, and only the desired channel is down-converted to baseband. In practice, however, the VCO output signal includes phase noise around the single tone at ω0. This phase noise further mixes with the received signal, which may result in the strong, undesired channel being down-converted to baseband if the bandwidth of the phase noise is larger than the distance separating the two channels (i.e., larger than Δf).
Assuming the bandwidth of the phase noise is larger than the distance separating the two channels, the undesired channel will interfere with the desired channel at baseband in the down-converted signal, reducing the sensitivity of the receiver. The effect of phase noise is similar for up-conversion. Therefore, for this reason and others, a VCO with low phase noise and a sharp output spectrum is desirable.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.